Rubber Diffuser Membranes vs. Ceramic Diffusers

By: Tom Frankel
Post Date: May 8th 2023

Supplying oxygen for aeration treatment processes is the single biggest energy use at any wastewater treatment plant. Typically, according to the EPA, supplying oxygen for aeration makes up 50% to 90% of the energy requirements of a wastewater treatment facility.

For this reason, choosing the right type of diffusers is crucial. A variety of different diffuser options are available for wastewater aeration systems. Two popular options are ceramic diffusers, which have a longer history of use, and the relatively new rubber diffuser membranes. We will outline some pros and cons of ceramic diffusers and rubber diffuser membranes below.

Rubber Diffuser Membranes

Rubber diffuser membranes have been in use in wastewater treatment facilities for about 40 years, and their technology has upgraded significantly in that time. Perforated diffuser membranes, in particular, have been developed over the last 10 or 15 years.

Rubber diffuser membranes come in two main types — tube diffusers and disc diffusers. Each disc or tube is perforated with dozens of tiny pores.

Tube membranes come in a variety of lengths, from 20 inches to 1 meter. Disc membranes range from 7 to 20 inches in diameter. However, with larger diameters, greater stress on the center of the membrane and the seal around the edge of the membrane is common, so smaller disc membrane diffusers are often recommended.

How Rubber Diffuser Membranes Work

Rubber diffuser membranes expand when air flows through them. Each tiny perforation on the diffusers is a variable orifice that expands to admit a bubble of air into the wastewater liquid. These air bubbles rise through the water column and transfer oxygen into the liquid to assist with aeration processes. Larger bubbles can also assist in the mixing process. When the airflow is turned off, the perforations close and prevent backflow of liquid into the aeration system.

Different diffusers have different perforation patterns. Disc diffusers can be punched in a circular pattern all the way around, and this punching pattern allows for low headloss and high oxygen transfer efficiency, or OTE. It also maximizes the number of perforations, but it puts a modest amount of strain on the membrane. It’s also common to punch a disc membrane in discrete perpendicular sections. Though this punching pattern increases headloss and lowers OTE, it puts less stress on the membrane.

Most disc diffuser membranes are punched with perforations of 1 millimeter. Holes smaller than 1 millimeter provide higher OTE but also raise headloss. Larger holes, conversely, lower headloss but diminish OTE. In Europe, tube diffuser membranes are likewise punched with perforations of 1 millimeter, but in North America, they are generally punched with perforations of 2 millimeters.

Materials in Rubber Diffuser Membranes

Rubber diffuser membranes are most commonly made from EPDM rubber, though diffuser membrane materials can also include polyurethane or silicone.

There are various benefits of silicone diffuser membranes vs. ceramic diffusers. The pros and cons of silicone diffuser membranes include the following. Silicone membranes offer superior chemical resistance and oxygen permeability. They, therefore, perform exceptionally well in fats, greases and oils. The soft composition of these membranes, however, means that they are very pliable and can easily tear.

Polyurethane diffusers also perform well in fats, greases and oils. These diffusers often operate with pulp, paper and food-waste products. But they do not perform as well as EPDM membranes in high temperatures, and they are hard to punch and seal.

Substances such as ash, carbon black, peroxides, plasticizers and organic additives are often included in rubber diffuser membranes’ composition to improve their performance. Adding peroxide to an EPDM membrane, for example, increases its tolerance of high temperatures, though it also reduces the membrane’s strength and may cause it to tear. Adding carbon black or other organic UV stabilizers provide UV ray protection.

Injection-Molded vs. Compression-Molded Rubber Diffuser Membranes

Rubber diffuser membranes can be either injection-molded or compression-molded membranes. To create injection-molded membranes, manufacturers pour molten rubber into a mold and let it cool. To make compression-molded membranes, manufacturers cut a roll into the shape of a disc.

Injection-molded membranes typically contain a higher plasticizer content — generally 30% to 35%. Compression-molded membranes, on the other hand, usually contain only 15% plasticizer. This lower plasticizer content helps avoid many of the issues associated with high volumes of plasticizer. At high volumes, the plasticizer often leaks out, and this leakage causes shrinkage, creep, hardening, cracking and lower OTE.

Compression-molded membranes also offer an advantage in terms of their uniformity. Injection-molded membranes typically only achieve 75% uniformity of material, whereas compression-molded membranes typically achieve 95% uniformity. This uniform composition is important for the membrane to function correctly.

Pros of Rubber Diffuser Membranes

Rubber diffuser membranes offer several advantages to wastewater treatment facilities.

1. They resist fouling. Particularly when treated with protective coatings like SSI’s patented PTFE and PEEK coatings, rubber diffuser membranes resist the buildup of unwanted sludge on their surfaces and pores.
2. They are less expensive to manufacture and install.
3. They incur fewer operating and maintenance costs. Their resistance to fouling means they will need less frequent cleaning and replacing.
4. They have a longer lifespan because of reduced fouling.
5. They can handle high loads, a high solids concentration and hard water. They can also maintain constant backpressure and high OTE even under these conditions.
6. Their variable orifices offer check valves to keep water and precipitates from entering the pipes when the diffusers are turned off.
7. They can be turned off. This is because they prevent the backflow of wastewater that would clog their pores and internal structure. For this reason, they work well for plants that need to turn their diffusers on and off regularly, are prone to power outages or want to keep their diffusers from becoming fouled during nitrification and denitrification.
8. They are energy efficient. Because wastewater diffuser treatment facilities can turn these diffusers off when necessary, the plants can save money on energy costs. Because they do not foul and increase backpressure, they also put less strain on the blowers, again leading to reduced energy costs.
9. They are ideal for waste where high concentrations of biosolids exist because they will not increase backpressure or reduce OTE in this situation.
10. They allow for greater turndown, so plants can achieve improved efficiency when operating at low airflow.

Cons of Rubber Diffuser Membranes

Rubber diffuser membranes can have several drawbacks as well, though most of these drawbacks are easy to neutralize.

1. They are prone to shrinking, creeping and cracking. Historically, this was true. The plasticizer in rubber diffuser membranes can seep out and cause the rubber membrane to harden and crack. New advances in technology, though, like protective seals and coatings, minimize these issues.
2. When new, they operate with more headloss than rubber diffuser membranes. This headloss can be controlled, though, by experimenting with orifice size and spacing.
3. They have an average lifespan of seven to 10 years, which is shorter than the lifespan of a well-maintained ceramic diffuser. Often, though, ceramic diffusers are not maintained to a level that makes their lifespan significantly longer than that of rubber diffuser membranes.
4. Basic, uncoated rubber diffuser membranes are not ideal for use with fats, greases and oils or with heat over 80 degrees Fahrenheit. However, plants can diminish these issues by using specially coated EPDM rubber membranes such as PTFE or FEPDM. These patented formulas coat the EPDM substrate in a protective sheath to resist the buildup of undesirable sludge, oil and grease. PTFE and FEPDM coatings increase rubber diffuser membranes’ lifespan and effectiveness in all types of heavy-duty wastewater by a factor of five to 10. With their reduced fouling and scaling, PTFE coatings are ideal for industrial wastewater from municipal plants, food-production facilities, oil and gas refineries, and pulp and paper mills, while FEPDM coatings offer additional enhanced chemical resistance.
5. When new, they have lower OTE than the average new ceramic diffuser. However, a recent study found that after six months of running in comparable conditions, new ceramic diffusers had fouled to the point that they had the same OTE as new membrane diffusers. In the study, the membrane diffusers had not fouled at all after six months and had not diminished in OTE.

Ceramic Diffusers

Ceramic diffusers are the oldest form of porous diffuser on the market. They have been in use widely in the wastewater treatment industry for many years but have lately run up against more sophisticated products.

Like rubber diffuser membranes, ceramic diffusers have many small holes through which air flows to aerate the wastewater liquid. Unlike the perforations in rubber diffuser membranes, the small holes in ceramic diffusers are fixed orifices, meaning they cannot open and close.

Material of Ceramic Diffusers

Most commonly, ceramic diffusers are made of alumina. Alumina, or aluminum oxide, a type of advanced ceramic, is formed from bauxite and vitreous silicate. Older generations of ceramic diffusers usually contained silicon dioxide, a material not quite as durable as alumina.

Ceramic diffusers can come in disc, tube, plate and dome shapes, though disc shapes are the most common. Typically, ceramic disc diffusers are 9 inches in diameter, though they can range from 7 to 20 inches in diameter. They can be as thin as 3/4 of an inch or as thick as 7 1/2 inches.

Pros of Ceramic Diffusers

Some of the benefits of ceramic diffusers as follows:

1. When new, they operate with less headloss than rubber diffuser membranes.
2. They will not shrink, creep or crack because they are made of fixed ceramic rather than rubber and plasticizer.
3. They perform better with fats, greases and oils and with heat over 80 degrees Fahrenheit
4. When new, they have slightly higher OTE than the average new rubber membrane diffuser. Increased susceptibility to fouling, however, can quickly erase this advantage.
5. They will last longer than rubber membrane diffusers if they are well maintained and do not become fouled. This is an academic point, however, because the vast majority of ceramic diffusers do become fouled.

Cons of Ceramic Diffusers

Ceramic diffusers also have several disadvantages, however.

1. They clog and become fouled. Their fixed orifices admit sludge, and they lack the protective coatings of rubber diffuser membranes.
2. They incur more maintenance costs because of their increased susceptibility to fouling, which means they require more frequent cleaning.
3. They are less energy efficient. Clogged and fouled diffusers lead to higher backpressure, which increases demand on the blowers, which in turn leads to higher power requirements and costs. Even a 10% increase in strain on the blowers can lead to plant energy costs that are 5% to 9% higher.
4. They are more expensive to manufacture and install.
5. They have shorter lifespans because of increased susceptibility to fouling.
6. They perform best in pristine conditions. Ceramic diffusers are at their best in aeration tanks with low loads, low solids concentration and soft water. This ideal environment rarely exists. Ceramic diffusers struggle to maintain steady backpressure and high OTE in suboptimal conditions.
7. They should not be turned off because their fixed orifices will become fouled in the absence of airflow. Therefore, they are not ideal for plants that turn their diffusers on and off regularly to save energy or during nitrification and denitrification.
8. They are not ideal for industrial wastes where high concentrations of biosolids would raise the backpressure and reduce the OTE.
9. They allow for less turndown, so plants achieve less efficiency when operating at low airflow
10. They are more expensive to clean.

Cleaning a Ceramic Diffuser System

Because of their susceptibility to fouling, ceramic diffuses are often sold with packaged gas cleaner systems that rubber diffuser membrane systems do not require. Hydrochloric acid and bleach also clean ceramic diffusers effectively. The EPA has found that the average cleaning returns ceramic diffusers only to 80 to 90% of their original performance. Also, once a ceramic diffuser pore becomes completely clogged, it is likely to remain that way despite subsequent cleanings.

The latest study of the cost of cleaning ceramic diffusers comes from the EPA in 1994. The EPA used data from Southern Methodist University, which had done reseach on cleaning such diffusers. The study found that the cost of removing a ceramic diffuser system and cleaning it ex situ by using pressure hoses and acid washes typically comes to over $3700.

Cleaning a ceramic diffuser system in situ involves specialized equipment and licensing fees. The same study found that the cost to clean a ceramic diffuser that processed 25 million gallons a day was $5000. This figure is low — the cleaning service provider was located relatively close to the treatment plant, only an hour away. The price would have been higher if the service provider had been farther away.

If a plant processing 25 million gallons a day operated 15,000 ceramic diffusers, and if that plant cleaned its diffusers once a month, it would pay about $76,000 in annual cleaning costs. Rubber diffuser membranes require much less cleaning and incur less expense.

How to Retrofit an Existing Aeration System

In recent years, membrane diffuser manufacturers have produced rubber diffuser membranes in standardized sizes. Practically, this standardization means that you can fit new membranes onto an old system without difficulty. Especially with SSI’s rubber diffuser membranes, which come in a wide variety of sizes, you are likely to find high-quality diffuser membranes that fit your needs.

SSI also supplies retrofit kits that contain all the equipment necessary for switching an existing ceramic diffuser system to a membrane diffuser system. So plant managers can rest easy knowing that nothing will be overlooked. To retrofit your existing ceramic system, SSI would examine the existing infrastructure using an in-depth checklist. Technicians would flag any potential issues and discuss solutions, and they would work with contractors and staff to train them on proper installation techniques and maintenance procedures.

To retrofit the old system, technicians would dewater and clean the tank and remove the old diffusers. They would install the membrane diffusers and new piping as necessary. They would also replace or upgrade the air filtration system as needed.

Often, the existing blowers can work with the new system. A plant would need to consider its current pipe installation and think about installing new piping to maximize the efficiency of the new rubber membrane diffusers. Sometimes new pipes can be installed at the same level as the old pipes. At other times, they may be installed higher off the floor to compensate for the higher headloss that may accompany the new rubber diffuser membranes.

The cities of Phoenix and Los Angeles recently retrofitted their existing systems to replace ceramic diffusers with rubber membrane diffusers. These successful operations show that retrofitting with rubber membrane diffusers can be an excellent option for modernizing a wastewater aeration system.

Retrofit Your Aeration System With SSI Aeration

SSI’s global service network is ready to assist you with retrofitting your existing aeration system with rubber diffuser membranes for superior performance. Contact us today.

Tom Frankel

Mr. Frankel co-founded SSI in 1995 with experience in design and distribution of engineered systems. He is in charge of sales, marketing and operations in the company. Mr. Frankel holds multiple US patents related to diffusers. He is a graduate of Washington University in St. Louis.